1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention relates to power supply circuits.
2. Background Art
Power supplies, such as switch-mode power supplies, can convert and regulate power from AC mains to desired DC voltages suitable for powering a load connected across the power supply. Standards and regulations imposed on these power supplies have become and inevitably will continue to become more stringent. For example, standards and regulations have lowered maximum standby power limits for power supplies in order to curb energy consumption. Thus, it is desirable to reduce power consumption when a load is disconnected from the power supply or is not in use. In doing so, a connection to the AC mains should be preserved to, for example, provide a low-voltage supply voltage to start up the power supply's control circuitry.
One conventional approach is ensuring that the low-voltage supply voltage of the power supply consumes low current during start-up of power supply control circuitry while continuously supplying this low current with a fixed high-ohmic resistor from the AC mains. However, this approach can no longer comply with maximum standby power limits as the resistor is continuously dissipating power after start-up. Another conventional approach uses a high-voltage start-up circuit that can connect to the AC mains to supply current to the power supply control circuitry when required, while otherwise disconnecting and withstanding the high-voltage. However, this approach requires complex and costly start-up circuitry including an additional large high-voltage start-up switch.
Thus, there is a need in the art for an improved start-up circuit for power supplies that overcomes the drawbacks and deficiencies in the art.